1. Field of the Invention
The present invention relates to an expandable stent.
2. Brief Description of the Prior Art
The term “stent” has been used interchangeably with terms such as “intraluminal vascular graft” and “expansible prosthesis”. Throughout this specification the term “stent” is intended to mean any expandable prosthetic device for implantation in a body passageway (e.g., a lumen or artery).
The use of stents has attracted an increasing amount of attention due the potential of these devices to be used, in certain cases, as an alternative to surgery. Generally, a stent is used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. As used in this specification, the term “body passageway” is intended to mean any duct (e.g., natural or iatrogenic) within the human body, including blood vessels, respiratory ducts, gastrointestinal ducts and the like.
Stent development has evolved to the point where the vast majority of currently available stents rely on controlled plastic deformation of the entire structure of the stent at the target body passageway so that only sufficient force to maintain the patency of the body passageway is applied during expansion of the stent.
Generally, in many of these systems, a stent, in association with a balloon, is delivered to the target area of the body passageway by a catheter system. Once the stent has been properly located (for example, for intravascular implantation the target area of the vessel can be filled with a contrast medium to facilitate visualization during fluoroscopy), the balloon is expanded thereby plastically deforming the entire structure of the stent so that the latter is urged in place against the body passageway. The amount of force applied is at least that necessary to expand the stent (i.e., the applied the force exceeds the minimum force above which the stent material will undergo plastic deformation) while maintaining the patency of the body passageway. At this point, the balloon is deflated and withdrawn within the catheter, and is subsequently removed. Ideally, the stent will remain in place and maintain the target area of the body passageway substantially free of blockage (or narrowing).
In the design of any new stent there are generally two functional constraints which govern the usefulness of the stent. First, the stent should have a high degree of flexibility in the unexpanded state. This is needed to facilitate navigation of the stent through tortuous anatomy to the location of the target stenosis. Second, the expanded stent should be radially rigid to minimize the effects of restenosis and the possibility of acute occlusion. Thus, an ideal stent would be characterized by divergent functional properties depending on the state of the stent (i.e., expanded or unexpanded).
Conventionally, the stent properties of flexibility in the unexpanded state and radial rigidity in the expanded state have been achieved using one set of interconnected struts (typically the longitudinal struts) to confer flexibility to the unexpanded stent and another pair of interconnected struts (typically non-longitudinal circumferential rings of struts) which open up to radially rigid hoop structures (in the ideal case) to confer radial rigidity to the expanded stent.
Unfortunately, this approach complicates the design exercise. Further, depending on whether the stent is in the expanded or unexpanded state, only a portion of the struts are being used (i.e., to confer flexibility or radial rigidity).
Accordingly, it would be desirable to have an improved stent which overcomes these disadvantages. It would be further desirable if the improved stent could be manufactured readily. It would be further desirable if the improved stent could be deployed using conventional stent delivery systems.